The present invention relates generally to the field of mechanical-strength test fixtures, and more specifically to a test fixture and method for assessment of joint strength of a multiple interconnection microelectronic component mounted on a printed circuit board or other substrate.
As electrical devices become both more capable and more popular, the market pressure to reduce costs while providing ever-more-reliable performance increases. This applies not only to computers and conventional cellular phones, but to devices such as personal digital assistants (PDAs), handheld electronic games, digital pagers, and Web phones as well. These devices are expected to provide reliable service notwithstanding the sophisticated technology they embody and their mobile nature. In addition, the instruments themselves are getting smaller. This is largely due to fast-paced technological innovation producing miniature electrical components, but also has the effect of forcing designers and manufacturers to abandon thick, sturdy enclosures in favor of more light-weight construction.
Notwithstanding the diminished ability of today""s electronic devices to withstand shock and bending stresses when compared to their bulkier predecessors, the size itself may contribute to rougher treatment. For example, custom carrying cases and automobile mounting for mobile telephones are no longer de rigueur; modern telephones may be simply hung from a belt, or stuffed into a pocket, purse, or briefcase. PDAs and even palm-top computers are subject to similar handling. And being smaller, they are sometimes easier to mishandle. But modern devices are expected to put up with such treatment.
What must actually withstand this somewhat rough treatment, of course, are the device""s internal components. Along with ways to make them smaller, scientists and engineers continually look for ways to improve the ability of electronic components to withstand the rigors of contemporary usage. As new strategies are tried, of course, new ways are developed to test their success at increasing mounted component""s shock-withstanding performance. As the present invention is directed to a fixture for performing such testing, a brief introduction to the components being tested will now be provided.
With the invention of the integrated circuit and the development of semiconductor technology, packaged chips replaced the tubes, wires, and mechanical switches of an earlier generation. A xe2x80x9cchipxe2x80x9d is a small piece of semiconducting material, such as silicon that has been xe2x80x9cdopedxe2x80x9d. Doping involves treating selected areas of the silicon with a doping agent such as boron, phosphorous, or erbium. These agents alter how the silicon will act electrically in the presence of an applied voltage or other stimulus. (Other types of treatment are used, as well.) Selective treatment of the silicon surface is often done through a process known as photolithography. In photolithography, a silicon wafer is covered with photoresist. A finely-tuned opto-mechanical device called a stepper exposes certain areas on the wafer to, for example, ultraviolet light, which cause some of the photoresist to harden (or soften) so that the selected portions may be washed away by an appropriate solvent. The partially exposed surface is then subject to doping, etching, or filling with a conductive material, and the remaining photoresist is removed. The process may be repeated any number of times to build the desired electrical components. The dozens of identical integrated circuits thus formed on the finished wafer surface are then separated from each other and encapsulated in plastic packages. These packages protect the chip and permit it to be transported and installed. Protruding through the package will necessarily be some form of conductor that connects the integrated circuit on the chip to the remainder of the electrical device in which it is to be used.
Component-manufacturing processes such as the one described above have resulted in smaller and smaller components that nevertheless offer greatly increased capabilities. The same is true for standard electrical components such as non-chip-integrated resistors and capacitors. These smaller components do not require study chasses to hold them in place, but can simply be mounted on a PCB made of non-conducting material and the whole assembly enclosed in a lightweight plastic containerxe2x80x94which also makes the device more portable. Although well-packaged, electronic components may nevertheless include many metallic connections to the printed circuit board (PCB). Board-mounted electrical components are used in a great many applications.
There are, generally speaking, three types of methods in common use for mounting electrical components to PCBs. First, the PCB may form an opening through which a component lead is inserted. A conductive trace on the PCB makes contact with the lead, which his usually soldered into place to ensure connectivity and to hold the component in place. Another component lead is attached through another opening in similar fashion. A single component may have only two or more than a dozen leads. In a second matter of attaching, the component has a plurality of relatively short leads, often called pins. The pins are inserted into sockets that are part of a previously installed terminal block. Finally, the electrical component may simply be attached to corresponding leads or traces present on the circuit-board surface. Such devices are often called surface-mount components and, since they use no through-hole or socket, must rely exclusively on the solder (or similar) joint to hold the component in place and to maintain each connection.
As previously mentioned, designers may wish to test the integrity of connections such as these to determine whether they are adequate and to plan for future improvements. As with many other mass-produced items, these devices are often tested destructively. That is, some adverse force or environmental condition, or both, is applied until the device fails. The device itself is thereafter unusable, unless repaired, but the critical condition causing failure is measured and recorded for comparison with other designs. Naturally, the accuracy of such tests is expected to improve as the number of sample tests increases.
Given the difficulties inherent in mechanically testing microcomponents, such testing frequently is performed on larger, isolated laboratory samples. For example, to estimate the shear strength of the mounting of an area-array mounted chip (See, e.g., FIGS. 1C and 2C), a single solder ball might be tested. The results are then extrapolated to approximate the expected strength of an entire installed. This nevertheless puts the designer at a disadvantage because the use of a single laboratory-sample solder ball may not very closely compare to an actual production-line installed chip.
At the other end of the spectrum, testing may be accomplished using the entire instrument, which also may be subject to various environmental conditions or simply dropped. When failure does occur, that is, the device becomes inoperable, the various components may be visually inspected or electronically tested to determine which ones contributed to the failure.
Note that as used herein, the terms electronic xe2x80x9cdevicexe2x80x9d, xe2x80x9cinstrumentxe2x80x9d, and the like will be used interchangeably to refer to manufactured products containing one or more electrical xe2x80x9ccomponentsxe2x80x9d or xe2x80x9cmicro-componentsxe2x80x9d to perform the designed function. xe2x80x9cMountingxe2x80x9d simply refers to the manner in which the component is fixed within the device. In the context of the present invention, this will often, but not necessarily, be done using a soldered connection. In short, the present invention is not limited in how it can be used, or on what specific components, save by whether they are of a construction that can be in some way placed into the fixture for testing.
Obviously, both the single solder ball test and complete-device test are useful, but unsatisfactory for determining the strength of a single mounted component. And conventional testing equipment is typically not suited to testing mounted components. Needed is a testing fixture capable of performing accurate strength tests on mounted components that closely resemble those used in the actual electronic device. The present invention provides just such a device, and a method of using it.
The present invention is directed to a test-fixture for testing mounted electrical components and to a method for using the fixture. In one aspect, the present invention is a test fixture including at least four plates stacked in a spaced relationship along a test-force axis; alternate plates being connected to one another via a plurality of alignment rods. The interior plates form openings so that the alignment rods connecting adjacent plates may pass through. Each of two loading arms are fixed to facing surfaces of two adjacent plates; the loading arms being positioned so as to apply a testing force to a specimen positioned between them when a force is applied by an external means to the outer plates.
In another aspect, the present invention is a method of testing mounted electrical components, including the steps of attaching the components to at least one of two loading arms, fixing the loading arms to adjacent plates in a test fixture having four plates, alternating plates being connected to each other by alignment rods. The method further includes the step of applying a force to the test fixture causing the loading arms to apply a force to the specimen.